bsd-user/mmap.c: check pread's return value to fix warnings with _FORTIFY_SOURCE

bsd-user/mmap.c: check pread's return value to fix warnings with _FORTIFY_SOURCE

Simmilar to the equivalent linux-user: commit fb7e378cf9c, which added
checking to pread's return value. Update to current qemu standards with
{} around the if statement.

Signed-off-by: Mikaël Urankar <mikael.urankar@gmail.com>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>